Magnetic disk storage systems are used widely to provide large volumes of relatively low-cost, computer-accessible memory or storage. A typical disk storage device has a number of disks coded with a suitable magnetic material mounted for rotation on a common spindle and a set of transducer heads carried in pairs on elongated supports for insertion between adjacent disks, the heads of each pair facing in opposite directions to engage opposite faces of the adjacent disk. The support structure is typically coupled to a positioner motor, and the positioner motor typically includes a coil mounted within a magnetic field for linear movement and oriented relative to the disks to move the heads radially over the disk surfaces to thereby enable the head to be positioned over any annular track on the surfaces. In normal operation, the positioner motor, in response to control signals from the computer, positions the transducer heads radially for recording data signals on or retrieving data signals from a pre-selected one of a set of concentric recording tracks on the disks.
The transducer heads are supported above the disk surfaces by a film of air to prevent contact therebetween which might thereby otherwise damage one or both members. The heads are typically designed to actually fly above the disk recording surfaces of heights less than 50 microinches. Irreparable damage can result from an electrical power failure which slows the disk and allows the head to settle into contact with the disk surfaces. As a result, it is imperative that the heads be withdrawn from the vicinity of the disk if the disk rotational speed is substantially reduced. It is also important in removable media disk drives to ensure that the heads are removed from the vicinity of the disk surfaces in event of power failure so that the disk can be removed from the system without damage to the heads.
The process of removing the heads from the disks in an emergency situation is referred to as an "emergency unload procedure" and requires the heads to be moved radially toward the disk's outer tracks axially away from the disk surfaces (OD). Although loss of power is probably the primary reason for initiating an emergency unload procedure, the procedure is typically also initiated when disk speed does not remain within tolerances, positional error is detected, or write circuits faults that could affect the stored data are detected.
Basically, all modern disk drives incorporate some assistance for executing an emergency unload procedure in order to avoid the loss of data and prevent disk and/or head damage. In typical prior art, emergency unload systems, a capacitor is charged by the drive power supply during normal operation. During the detection of an emergency condition, a relay or equivalent switching means switches the capacitor across the positional coil terminals to provide the electromagnetic force necessary to move the.head support structure across the disk surfaces. Upon approaching the disk's outer edge, the head support structure encounters a mechanical ramp which imparts an axial force to the support structure, thus unloading the heads from the disk.
FIG. 1 illustrates a prior art system, which includes a three-phase spindle motor 130 which drives the disks, and a drive circuit 116, which is used to control the commutation of motor 130 during normal operation. As illustrated in FIG. 1, the drive circuit 116 includes a plurality of FET circuits which have a inherent set of diodes numbered 110, 112 and 114 across the source to drain of each FET. In addition, to connect to the VCM (voice control motor) 120, a set of Schottky diodes, numbered 102, 104 and 106, connect with capacitor 100, which is connected in parallel with the VCM motor 120. In operation, the back-EMF voltage from motor 130 is fed to Schottky diodes 102, 104 and 106, respectively, and from the Schottky diodes 102, 104 and 106, the back-EMF voltage is fed to capacitor 100. The three Schottky diodes 102, 104 and 106 perform passive rectification to allow the back-EMF voltage to charge the capacitor 100, and this charge stored on capacitor 100 is used to power the VCM motor 120 during emergency conditions.
However, the voltage produced by the motor 130 is typically very low on the order of 3.5 volts peak-to-peak in mobile servo application. During emergency conditions, the back-EMF voltage is rectified by the beforementioned diodes, and consequently, as a result of the rectification, the back-EMF voltage is reduced to approximately 2.2 volts peak-to-peak. Further losses occur as a result of the voltage drops across two diodes in series on the VCM retract current path, resulting in only approximately 0.7 or 0.8 voltages being applied to the VCM motor 120. As a consequence, the current available to move the heads across the disk surface and out onto the ramp is significantly reduced. A large amount of current is required to park the head on the ramp and this is not sufficient for any true ramp load applications.
FIG. 2 illustrates another circuit used to convert the back-EMF voltage to energize the VCM motor 220. This circuit includes a set of bipolar transistors 202, 204, 206, 208, 210 and 212 that are used to control the back-EMF current. However, this circuit suffers from two additional defects. First, depending upon a particular design, the finite value of V.sub.BE,ON may vary significantly with process, thus resulting in a deadband where no rectification occurs and no back-EMF voltage is available. Additionally, the circuit in FIG. 2 requires a discrete logic circuit 222 to control the bases of transistors 202, 204, 206, 208 and 212. The bipolar circuits are not compatible with CMOS process, and, as a consequence, the circuit must be external to the circuit (assuming a CMOS circuit) used to drive the motor. Thus, consequently, there is a need for a circuit which is economical in terms of current to operate and is highly accurate.